A temperature monitoring circuit has many applications. One crucial application for temperature monitoring circuitry is to protect electronic components from being subjected to high temperatures. Cellular phones, laptop computers, and other battery-operated powered electronic devices have numerous electronic components which can be easily damaged by high temperatures. For example, transistors in integrated circuits in battery-operated powered electronic devices are known to breakdown at high temperatures.
Typically, a temperature monitoring circuit utilizes the properties of a bipolar transistor. In operation, a temperature independent reference voltage is generated by use of a band-gap circuit, a known technology in the field. The reference voltage is compared with a temperature-dependent voltage. When a certain criterion is met, a voltage signal is produced to indicate excessive temperature.
U.S. Pat. No. 4,887,181 to Lenz describes a temperature protection circuit that applies the fundamental concepts of a band-gap circuit and a bipolar transistor. The temperature protection circuit amplifies the reference voltage from the band-gap circuit by an amplifier circuit and delivers the amplified voltage to a sensor circuit which compares the amplified voltage with a temperature-dependent voltage generated by the sensor circuit. The temperature protection circuit also includes a positive feedback loop from an output of the amplifier circuit back to an input of the amplifier circuit. The sensor circuit is connected to a bipolar transistor which is turned on when the comparison of the amplified voltage with the temperature-dependent voltage indicates that a set temperature has been reached. A current flow caused by the activation of the bipolar transistor causes a power amplifier, the electronic component that is being protected, to shut off. The temperature protection circuit also exhibits a hysteresis characteristic due to the sensor circuit being thermally coupled to the power amplifier.
U.S. Pat. No. 4,789,819 to Nelson utilizes a specific band-gap circuit, i.e. a Brokaw Cell band-gap circuit, to provide biasing for a thermal shutdown circuit. The Nelson circuit modified the basic Brokaw Cell band-gap circuit by including a breakpoint compensation. The basic Brokaw Cell band-gap circuit generates an output voltage that is nearly constant with temperature variations. However, the fluctuation of the output voltage increases with greater temperature variations. The breakpoint compensation of Nelson shifts a temperature coefficient of the circuit to decrease the voltage fluctuation over a certain temperature range. The circuit, similar to the circuitry described in the Lenz patent, includes a bipolar transistor that turns on when a predetermined temperature is reached and the base of the bipolar transistor is thereby properly biased.
While the two disclosed circuits operate well for their intended purposes, there are concerns regarding their limitations. For circuitry that includes complementary metal-oxide semiconductor (CMOS) transistors, a temperature protection circuit utilizing bipolar transistors is very difficult to manufacture due to the lack of general purpose bipolar transistors.
What is needed is a temperature protection circuit that utilizes standard CMOS integrated circuit technology processing without the need to fabricate special bipolar transistors.